1. Field of the Invention
The present invention relates to a circuit for preventing transmission of a fixed pattern of an optical digital transmission equipment, and more particularly, to a circuit for preventing transmission of a fixed pattern of an optical digital transmission equipment which eliminates a problem caused, when an electric signal to be multiplexed by an overhead bit necessary for optical digital transmission has a fixed value, by the generation of a direct current component at an O/E conversion unit for converting the optical signal into an electric signal.
2. Description of the Related Art
Well-known as large-volume transmission is multi-channel•time-divisional optical digital transmission. Techniques related to the optical digital transmission are recited, for example, in Japanese Patent Laying-Open (Kokai) No. Showa 63-300642, Japanese Patent Laying-Open (Kokai) No. Heisei 3-244238 and Japanese Patent Laying-Open (Kokai) No. Heisei 5-199199. The literature recites that a multiple conversion device necessary for optical digital transmission needs comparison between a phase of a write clock of a low-order group input signal for writing data into a memory which temporarily stores data and a phase of a read clock of a high-order group signal for reading the data therefrom and control of these phases.
FIG. 3 shows one example of a conventional optical digital transmission equipment having such a phase comparator.
A low-order group signal 101 is temporarily accumulated in a memory 102. Transfer of a write address 105 to the memory 102 by a write address counter 103 based on a low-order group signal clock 104 results in that the low-order group signal 101 is written in the memory 102. Transfer of a read address 108 to the memory 102 by a read address counter 106 based on a high-order group signal clock 107 results in that a memory output signal 109 is read from the memory 102.
The read memory output signal 109 has an overhead bit necessary for optical digital transmission multiplexed at a multiplexing circuit 110. A multiplexed signal 111 output from the multiplexing circuit 110 is converted into an optical signal at an E/O conversion unit 112 and transmitted to an O/E conversion unit 114 on the reception side through an optical fiber 113.
As is already described, a phase comparator 115 is provided. The phase comparison circuit 115 monitors a phase difference between a phase of write to the memory 102 and a phase of read from the memory 102 and when the low-order group signal clock 104 gets so much out of order because of disturbance etc. that a memory phase difference enters a range in which memory slippage occurs, the circuit considers it as overflow or underflow and outputs a first reset signal 116 in order to forcibly return the memory phase difference to an appropriate value.
Data accumulated at the memory 102 is cleared by the first reset signal 116. As a result of this clearance, the memory 102 outputs a fixed value “1” which is a memory initial value during a period from when the low-order group signal 101 is newly written in the memory 102 until when the same is newly and normally read.
Since a time period where such a fixed value “1” is continuously output is increased in proportional to a memory capacity of the memory 102, the larger the memory capacity is, the larger a fixed value continuous output time becomes. The multiplexed signal 111, that is, a digital signal multiplexed and then output by the multiplexing circuit 110 to which such a fixed value is applied has its mark rate (duty ratio) not attaining 0.5. Mark rate failing to attain 0.5 represents that a direct current component is generated.
When such a direct current component is generated, such an unfavorable phenomenon occurs as a discrimination error that a discriminator 118 of the O/E conversion unit 114 including a capacitor 117 erroneously discriminates a reception signal, so that a clock extraction circuit 119 fails to extract a clock of the reception signal to result in causing stop of an output of a clock output signal 120. As another unfavorable phenomenon, when such a fixed value signal is used in an optical wavelength multiplex transmission system, mean power of an optical signal in the system changes to affect a signal of other wavelength.
Technique for suppressing generation of such a direct current component is known. According to the well-known technique, a scrambling circuit is disposed preceding to an E/O converter and a descrambling circuit is disposed succeeding to an O/E converter. This technique has a problem that when transmission is conducted at a super-high speed, it is difficult to provide a scrambling circuit and a descrambling circuit in terms of device designing, resulting in increasing a circuit scale.
Suppressing generation of such a direct current component is demanded. Further demanded is simplification of a circuit for suppressing generation of such a direct current component.